In response to a market demand for the ability to transmit data, typically over relatively short distances, local area networks (LANs) have been developed. LANs are telecommunications networks which interconnect a plurality of stations which are typically located in a relatively compact geographical area. In contrast, the long distance telecommunications network spreads over an enormous geographical area. The differences, e.g., geographical area, between the two types of networks frequently lead to design differences. For example, the physical location of nodes, i.e., a point where two or more fines intersect, is determined primarily by geography in long distance networks but in LANs the location of nodes is frequently determined by other considerations, such as the frequency of communications between two stations, i.e., how frequently two stations talk to each other, transmission medium, transmission delays, etc., rather than by geography.
In attempts to optimize the efficiency of LANs, different topological configurations have been developed. These configurations include star, ring, bus, and tree. A discussion of all of these topologies will not be given because it is not required to understand this invention. Ring or loop systems will be discussed because they are relevant to the invention. A loop system has a plurality of serially connected stations in a ring. For a description of such a system see, e.g., U.S. Pat. No. 4,293,948 issued on Oct. 6, 1981. The loop configuration is simple and straightforward because each station is connected directly to the loop, but consideration of the requirements for efficient operation shows that the system should have an orderly means of determining which station transmits information and which station receives the information. One means that has been developed is termed the token. During system operation, a station waits until it receives the token and transmits only while it possesses the token. The addressed station copies the information while nonaddressed stations regenerate the signal and transmit it to the next station on the loop. The originating station removes the information from the loop after one trip around the loop. Token rings may be either synchronous or asynchronous.
The loop system described is unidirectional and consequently suffers several drawbacks, including single paths between two stations, thereby rendering the system subject to catastrophic failure and decreased efficiency if some portions of the loop are used more extensively than are other portions. To overcome these drawbacks, as well as for other reasons, bidirectional loop systems, such as the fiber distributed data interface, have been developed. This system, commonly referred to by the acronym FDDI, is a token ring network which uses two rings that are referred to as the primary and the secondary ring. Information flows in opposite directions in the two rings, but only one loop is typically used in the absence of faults. The stations are designated as either Class A or Class B stations. Class A stations have two inputs and two outputs with one out of each connected to the primary ring and to the secondary ring. Class B stations have a single input and a single output and are connected only to the primary ring. Each station can identify faults in the rings and initiate steps so that the network reconfigures itself and remains in operation,: i.e., the system uses distributed control.
While this system is advantageously employed in many situations, it and other loop systems suffer from several limitations. One of the two rings is generally used more than the other ring, thus lowering system utilization. Furthermore, each station has means for switching packets, i.e., a defined set of bits, between the primary ring and the secondary ring which is generally used only if faults are present. Thus, this switching means, as well as the means for transmitting packets on the secondary loop, is not much used in the absence of faults. Also, any two stations are connected by a very limited number of possible paths, and the number does not increase as the number of stations increases. Furthermore, transfers between loops are accomplished with apparatus termed bridges which occupy positions on the loop but perform no station functions.
Several of these limitations can be avoided by the use of mesh networks. Such networks have pluralities of intersecting horizontal and vertical lines, i.e., paths, with a station or other apparatus located at the points of intersection or nodes. A mesh network thus has a plurality of point-to-point communication paths or channels. Mesh networks are advantageously employed, as compared to loop networks, because they provide numerous alternative paths between stations, thus making the network more resistant to faults than are loop systems and providing alternative routes in times of heavy usage. One mesh network is termed the Manhattan Street Network and is commonly referred to by the acronym MSN. See, e.g., the article by Maxemchuk in A.T.&T. Technical Journal, pp. 1659-1685, September 1985 for a detailed description of an MSN. The name MSN is descriptive of the network because it is a mesh with a first plurality of paths running horizontally and a second plurality of paths running vertically. The first and second pluralities are referred to as streets and avenues, respectively, because streets and avenues in the city of Manhattan run generally east-west (horizontal) and north-south (vertical), respectively. Each point where the first and second pluralities intersect is a node with two inputs and two outputs. Each packet is identified by two coordinates (one for the row and one for the column) for the source and by two coordinates for the destination. The MSN is a relatively efficient system because there is a choice of routes between the source and destination, thereby permitting both faulty routes and busy nodes to be avoided.
The relatively economical use of system resources and ease of switching present in MSN networks has not been realized in token loop systems.